The ever-increasing demand for higher and higher disk storage capacity has required that the packing density of the bits be increased. With such increases the signal to noise ratio decreases, requiring ever more precision in the read out process to insure that the bits are discriminated from the noise and no information is lost. Writing into a magnetic disk is accomplished by representing each binary "1" as a transition either positive or negative going, while a "0" is represented by no transition. In the read operation a clock is regenerated and the transitions are read out synchronously with the clock signal. The noise that occurs can be reduced in one approach by thresholding the signal so that under a certain level transitions are ignored as noise. In another approach any transition which is not of the opposite polarity of the previous one is ignored as noise.
Quite apart from analog attempts to discriminate the signal from the noise, there are digital approaches too. For example, the signal read from the disk can be sampled and held and digitized. The digitized signal can then be synchronized with a regenerated clock signal. Then the signal will only be viewed at those precise clock intervals and only transitions occurring at those times will be recognized as signal or data. This approach is quite complex and costly because it requires a sample and hold circuit, an analog to digital converter and full digital processing circuits.
One of the more sophisticated digital processing approaches known as Viterbi detection tracks the path of previous decisions and calculates a running total of path metrics for use in discriminating and detection. Another approach uses multiple threshold levels to more precisely derive qualification or threshold levels for the positive and negative going peaks which represent binary "1"'s.